1. Field of the Invention
This invention relates to rechargeable fuel cells and more particularly to an improved cell in which the positive electrode contains the oxidizing agent and the negative electrode is formed from a hexagonal intermetallic compound of the composition AB.sub.5 where A represents a rare-earth metal and B represents nickel or cobalt.
2. Prior Art
Considerable interest and attention has been directed to the development of fuel cells and much research has centered around problems of safety due, primarily to the use of high pressure hydrogen in lightweight pressure vessels. Optimizing the cells while maintaining or increasing efficiency has been the subject of continuing study with the development of a type of cell using a gas as one member of the electrochemical couple and a chemically active solid state material as the other member. A survey of contemporary research efforts in this area is found in a 1973 publication by NASA entitled, "Fuel Cells", SP-5 NASA SP-5115.
As disclosed in U.S. patent application Ser. No. 259,524, filed on June 5, 1972 now U.S. Pat. No. 3,867,199 and entitled "Nickel Hydrogen Fuel Cell" (and assigned to the assignee of this invention), one type device is a fuel cell wherein at the negative electrode is a chemically oxidizable and ionizable gas such as hydrogen and at the positive electrode is an electrochemically reducible metal compound typified by nickel hydroxide. In accordance with the above referenced invention, the positive electrode is nickel hydroxide on a conductive support and the negative electrode comprises a catalytic layer of platinum or palladium on a conductive support. Between these electrodes is a separator wetted with an electrolyte such as an aqueous solution of KOH.
Cells of this type can operate over a wide range of ambient temperatures and can be constructed in various configurations with inherent overcharge and overdischarge protection. The cell, however, must be hermetically sealed after filling with hydrogen and typically operates at pressures ranging from 100 to 600 psia at room temperature. Accordingly, special design considerations are present in this type of cell to effectuate operation in this high pressure realm. Furthermore, since hydrogen is stored as a gas, extreme care must be exercised to avoid explosions caused by hydrogen leakage. Some of the operating problems and conditions for these cells are discussed in Earl and Dunlop, "Chemical Storage of Hydrogen in Ni/H.sub.2 Cells", COMSAT TECHNICAL REVIEW, Fall, 1973.
Accordingly, studies have tended in the direction of attempting to define systems which will store hydrogen as a reduced compound rather than as a gas at higher pressures. Some hexagonal intermetallic compounds of the generalized composition AB.sub.5, where A represents a rare-earth metal and B represents nickel or cobalt, are known to easily absorb and desorb large quantities of hydrogen gas under relatively small pressures at room temperature. The ability of these compounds to absorb hydrogen is described in van Vucht et al. "Reversible Room-Temperature Absorption of Large Quantities of Hydrogen by Intermetallic Compounds", Philips Research Reports, Vol 25, pp. 133-140 (1970). This property of hexagonal nickel-rare earth metal compounds was utilized in U.S. patent application Ser. No. 506,086 now U.S. Pat. No. 3,959,018, "Low Pressure Nickel Hydrogen Cell", (assigned to the same assignee of the present application) which describes and claims the use of LaNi.sub.5 for the chemical storage of hydrogen in Ni/H.sub.2 cells. As shown in that patent application, the hydrogen absorbing compound is stored in a hermetically sealed pressure resistant chamber comprising the cell and is separated from the electrode stack. Cells constructed in the manner taught by that application operate in a pressure range of 15 to 30 psia at room temperature with a maximum pressure in the order of 45 psia. It is evident that such reduced pressures make the design of the cell itself easier as well as eliminating the major safety hazard, that of high pressure gaseous hydrogen since it is now stored as a reduced compound instead of as a gas.
Despite the improvements represented by the above referenced patent application, such a cell when chemically storing hydrogen requires an intermediate absorption or desorption step prior to discharge or charging. In the case of lanthanum nickel hydride, the reactions are represented by the equation: ##STR1## The hydrogen gas upon reaching the surfaces of the catalyst of the negative cell plates dissociates by the action of the catalyst to monatomic form and from this point the reactions shown below are conventional to fuel cells. ##STR2##
The present invention eliminates this principle disadvantage of the prior art by using LaNi.sub.5 as the negative electrode in the fuel cell. The prior art, such as represented by two patents to Dilworth, U.S. Pat. Nos. 3,405,008 and 3,405,009 discloses the use of intermetallic compounds as fuel cell electrodes but neither patent teaches the use of an intermetallic hydride. The U.S. Pat. No. 3,405,008 discloses the use of a generalized compound MNi.sub.5 (wherein M is a rare earth) and the U.S. Pat. No. 3,405,009 teaches a compound M'Ni.sub.3 wherein M' is a transition metal. Similarly, U.S. Pat. No. 3,669,745 to Beccu discloses an accumulator electrode comprising nickel and a mixture of titanium hydride or zirconium hydride and the hydrides of the rare earths (Col. 2, lines 52-56). There is no suggestion in this patent that the electrode comprises an intermetallic compound, although in Col. 3, lines 53 et seq. it is indicated that there is some alloying between the metal hydride and the activating material.
Accordingly, it is an object of this invention to use an intermetallic hydride as an electrode in a fuel cell.
It is another object of this invention to contain the hydrogen fuel in a compact solid hydride form thereby eliminating the need for high pressure vessels to contain the fuel cell.
It is still another object of this invention to replace the platinum electrode currently being used for the negative electrode with one of a series of intermetallic hydride electrodes.
A further object of this invention is to eliminate the intermediate step of absorption or desorption required where an intermetallic hydride is stored separately from the fuel cell electrodes.
Yet another object of this invention is to reduce the volume of a fuel cell as compared with either nickel-hydrogen or nickel-cadmium cells.